1. Field of Invention
This invention relates to plasma processing systems, and more specifically, to improved methods and systems for multi-feed RF power distribution in plasma processing systems.
2. Description of Related Art
Plasma enhanced chemical vapor deposition systems are used to fabricate silicon thin films such as amorphous silicon (a-Si), microcrystalline silicon (μc-Si), silicon oxide (SiO2), and silicon nitride (Si3N4) for thin film transistors (TFTs) and solar cell, i.e., photovoltaic, applications. Generally, a substrate is supported in a vapor deposition process chamber and is heated to several hundred degrees Celsius. The substrate may be made of glass, quartz, or a polymer. The substrate size can be, for example, 650 by 830 millimeters, although the trend is toward even larger sizes. Deposition gases are injected into the chamber, and excited into a plasma state by a RF (alternating current) frequency between two or more electrodes driven by a power system. A plasma enhanced chemical vapor deposition reaction occurs to deposit a thin film layer onto the substrate. The deposited thin film layer may be a dielectric layer, such as silicon nitride or silicon oxide, or a semiconductor layer, such as amorphous silicon.
Large area deposition is essential to achieve high fabrication throughput, however, maintaining uniform film thickness and characteristics across the substrate becomes increasingly difficult with larger deposition areas. This is due to the formation of standing waves as the wavelength of the excitation frequency approaches the physical dimensions of the electrode.
A number of solutions focusing on the electrode configuration have been proposed for improving deposition uniformity in large plasma chambers relative to the wavelength of the applied RF in the chamber. However, these solutions are generally not capable of delivering a uniform electromagnetic field at a given time. Only the time-averaged value of the magnitude (and/or the magnitude raised to some exponential power) of the electromagnetic field can be made relatively constant over the dimensions of the chamber through some form of modulation of the field (e.g., by phase modulation or mechanical movement).